A police officer killed by a Co Down dissident republican just across the border was shot dead with a high-powered Glock pistol of a type preferred by gangland assassins and special forces.

Investigators discovered the 9mm Glock automatic beside the body of killer Adrian Crevan Mackin, who took his own life after shooting his partner and hero Garda Tony Golden who was trying to protect her.

Sources said that there was still a number of bullets in the magazine - which the violent terror suspect would have used to shoot anyone else who came into the house.

Last night his partner Siobhan Phillips was still in a critical condition in Beaumont Hospital in Dublin, being treated for a gunshot wound to the head.

"This was a high-powered weapon and there were more unused rounds in the magazine - if other gardai or the woman's father had entered the house it is likely that they too would have been shot," a source revealed.

"The thinking is that he would have kept on shooting until he got everyone and then taken his own life - he had plenty of ammunition."

Investigators believe that it is likely to have come from a cache of weapons in the possession of the IRA, which has used Glock automatics for assassinations and shootings in the past.

Meanwhile, it also emerged yesterday that 24-year-old Crevan Mackin, who was originally from Rostrevor, Co Down, had been assaulting his partner before they moved recently from Newry to the house where the horrific murder took place on Sunday evening.

It is understood that the family of the young woman were deeply concerned for her safety and wanted her to leave him.

Yesterday more details of the appalling shooting began to emerge as gardai continued to examine the scene.

On Friday Mackin's partner went to the local gardai in Omeath to report that she had been assaulted and arranged to meet Mr Golden on Sunday to make a statement.

It is understood that the young woman, who has a child with the killer, wanted to collect some clothes and belongings at the house in the Mullach Alainn estate in the village before going to stay with her parents.

At this stage, sources say that the belief was that Mackin was not in the house.

But Garda Golden accompanied the woman to the house to protect her - and instructed her father to stay in the car.

Colleagues say that this decision probably saved the man's life.

Investigators believe that the woman was shot first and then Mr Golden whose body was found on the stairway.

It is believed that he moved to intervene when the shots were first fired, again a decision which likely saved the life of Siobhan.

Her grandfather John Philips last night described the incident as an "awful tragedy" and spoke of the family's despair from his home in Jonesboro.

He said: "She's still critical. It was an awful tragedy. All we can do now is hope. At this stage my head isn't settled enough to say anything else."

Garda Commissioner Noirin O'Sullivan spent most of the day in Dundalk and Omeath yesterday, where she visited the murdered officer's grieving widow Nicola and colleagues.

"Today is a very sad and tragic day for members of An Garda Siochana and indeed the local community here in Dundalk," she told a press conference at Dundalk station.

"I'm particularly mindful that this is the second tragedy that has been visited on the local community here and in particular the members in Dundalk district," she added.

It is less than three years since Detective Garda Adrian Donohoe was shot dead in Co Louth during a robbery.

Ms O'Sullivan said: "It is a very traumatic time... there is one lady who is in hospital. We wish her a speedy recovery, we understand at the moment she is in a critical condition.

"This is a very tragic event, to lose a member of An Garda Siochana.

"Our thoughts are with the families of the now 88 members who have given their lives in the line of duty."